Do you two have any USB drives connected to the USB port??

What browser are you using to access the router?
@zbeyuz - What region are you located?
What wireless neighbors do you have near by? How many?
After updating FW, both wireless have been stable for me.
I'll reconnect my 5500 and see if I notice any issue. I'll do some gaming with my xbox on 2.4Ghz. Granted there are only a few other Wifi nieghbors in my area and I also use manual channels on my routers.
As for FW updates and frequency, I don't agree with this and rather have the engineers and developers take there time in making changes, fixing what needs to be fixed and properly testing the fixes and other issues, then rushing to attempt to just fofill a few users complaints about problem that may or may not be FW related.
Everyone needs to show some patience and let D-Link work out the real problems and issues. If your not willing to be patient then you should probably return the router for a refund and find other alternatives and work better for you.